The need to seal sewn seams to provide waterproofness has been known for many years. Early approaches to solve this problem involved the application of rubber solutions in volatile solvents, or adhesively bound strips of rubber to the seams. Later, commercially available synthetic latex dispersions were used for the same purpose with rubberized cloth. Such method are still in use.
With the advent of breathable waterproof fabrics, that is, fabrics which repel liquid water but allow the passage of water vapor, and their use in outer garments, the same problems of seam sealing arose again. Initially, welding the outer fabric was tried instead of sewing. Welding was slow and the seams were generally not as strong as sewn seams and it was found the welded seams were not necessarily waterproof. Later, various pastes in volatile solvents were painted on the seams, but this was messy and the rate of failure at the seams was far too high. Seam sealing tapes were adopted to solve the leakage problem.
To provide a durable waterproof sewn seam, it is necessary to enclose and seal the sewing thread and the ends of the fabric at the seam with a waterproof material. Sealing is accomplished using a thermoplastic hot melt adhesive. It is necessary to ensure that the hot melt adhesive penetrates the fabric all of the way through the fabric to the material which supplies waterproofness and breathability to the garment on both sides of the loose edges of the sewn seam. Seam tape is optionally used on either the lining fabric or shell fabric or directly on the material providing waterproofness.
On the surface of the garment where two seams intersect, it is necessary to have the sealing tape on one seam cross over the sealing tape on the second seam.
The structure of sealing tapes, especially those for use with breathable fabric laminates utilizing expanded porous polytetrafluorethylene membrane as a breathable liquid barrier, usually contain at least two layers; the first layer is most commonly a thermoplastic hot melt adhesive and the second layer is a waterproof layer which can be expanded porous polytetrafluoroethylene or a high melting or non-melting film layer. The film may be any of a number of plastic materials. Optionally, one may also have a third layer of fabric in the seam tape.
The choice of materials for the thermoplastic hot melt adhesive is fairly wide. Thermoplastic films of polyester, polyamides, fluoropolymers and polyurethanes are all used commercially, as hot melt adhesives.
Due to the great difference in melt point, tapes containing an expanded porous polytetrafluoroethylene layer have an advantage of being more dimensionally stable during application than those without said layer. Application of the tape over a seam involves heating the thermoplastic hot melt adhesive, usually in a directed blast of very hot air, up to 800.degree. C., although the tape does not achieve anything like that temperature. High air temperatures are used to accelerate the seam sealing operation and the residence time of the thermoplastic hot melt adhesive exposed at such temperatures is very short. Immediately after being exposed to the hot air, the fabric with the tape over the seam is fed into a compression zone to press the softened thermoplastic hot melt adhesive layer into the fabric. The very nature of the operation induces intermittent motion which, if the tap is not stabilized, i.e. by a layer of expanded porous polytetrafluoroethylene, the tape will tend to stretch and narrow when heated. Without a dimentionally stable expanded porous polytetrafluoroethylene layer, the threads of the fabric have a tendency to push through the tape while in the compression zone and, as a result, initial or long-term waterproofness is sacrificed.
In many cases, it is desirable to use two-layer seam tapes, which are less bulky and produce sealed seams with lower contours. However, two-layer seam tapes manufactured with expanded porous polytetrafluoroethylene films adhered to thermoplastic hot melt adhesive suffer form a distinct blocking problem. That is, when stored under tension in rolls, prior to use, and especially during the hot summer months, the thermoplastic hot melt adhesives tend to cold creep or slowly flow into the pores of the adjacent layer of expanded porous polytetrafluoroethylene creating bonds between the adjacent layers of tape. These bonds formed from the blocking phenomenon can develop strength sufficient to result in destruction of the adhered expanded porous polytetrafluoroethylene layer when the tapes are unrolled and stripped apart. It is to this blocking in storage problem that this invention is addressed.